Switch mode power supply controllers

ABSTRACT

This invention generally relates to discontinuous conduction mode switch mode power supply (SMPS) controllers employing primary side sensing. We describe an SMPS controller which integrates a feedback signal from a point determined by a target operating voltage to a peak or trough of an oscillatory or resonant portion of the feedback signal when substantially no energy is being transferred to the SMPS output. When regulation is achieved this value should be zero; the difference from zero can be used to regulate the output voltage of the SMPS.

RELATED APPLICATION

This application claims priority under 35 U.S.C. 119 from United KingdomApplication No. 0610206.5 filed 23 May 2006, which application isincorporated herein by reference.

FIELD OF THE INVENTION

This invention generally relates to a switch mode power supply (SMPS)controllers and to related methods, and more particularly to SMPScontrollers employing primary side sensing.

BACKGROUND TO THE INVENTION

Broadly speaking in a switch mode power supply a magnetic energy storagedevice such as a transformer or inductor is used to transfer power froman input side to an output side of the SMPS. A power switch switchespower to the primary side of the energy storage device, during whichperiod the current and magnetic field builds up linearly. When theswitch is opened the magnetic field (and secondary side current)decreases substantially linearly as power is drawn by the load on theoutput side.

An SMPS may operate in either a discontinuous conduction mode (DCM) orin continuous conduction mode (CCM) or at the boundary of the two in acritical conduction mode. In this specification we are particularlyconcerned with DCM operating modes in which, when the switching deviceis turned off, the output voltage steadily, but gradually, declinesuntil a point is reached on the knee of the output curve at whichsubstantially zero output current flows an the inductor or transformerbegins to ring, entering a so-called oscillatory phase. The period ofthe ringing is determined by the inductance and parasitic capacitance ofthe circuit. In this specification DCM includes so-called critical(discontinuous conduction) mode (CRM) operation in which the powerswitch is turned on again at the first trough of the oscillatory phase(sometimes referred to as the flyback oscillation). Operation in CRM canbe particularly efficient by reducing losses associated with the powerswitch turn-off transition.

Often the output voltage of an SMPS is regulated by sensing circuitry onthe output side, coupled back to the input side of the SMPS by means ofan opto-isolator. However some improved techniques employ primary sidesensing or, more generally, sensing employing an auxiliary winding onthe magnetic energy storage device, or in some related circuits anauxiliary winding of an output filter inductor.

Some background prior art relating to primary side sensing can be foundin U.S. Pat. No. 6,958,920; U.S. Pat. No. 6,721,192; US2002/015315; WO2005/048442; WO 2004/051834; US2005/0024898; US2005/0169017; U.S. Pat.No. 6,956,750; U.S. Pat. No. 6,862,198; US 2006/0056204; U.S. Pat. No.7,016,204; US 2006/0050539; US 2006/0055433; US 2006/0034102; U.S. Pat.No. 6,862,198; and U.S. Pat. No. 6,836,415. Still further backgroundprior art can be found in U.S. Pat. No. 6,385,059, US20050276083, U.S.Pat. No. 6,977,824, U.S. Pat. No. 6,956,750, U.S. Pat. No. 6,900,995,WO2004082119, U.S. Pat. No. 6,972,969, WO03047079, U.S. Pat. No.6,882,552, WO2004112227, US2005285587, WO2004112226, WO2005011095, U.S.Pat. No. 6,985,368, U.S. Pat. No. 7,027,312, U.S. Pat. No. 6,373,726,U.S. Pat. No. 4,672,516, U.S. Pat. No. 6,301,135, U.S. Pat. No.6,707,283, and U.S. Pat. No. 6,333,624.

Referring now to FIG. 1, this shows an example of a switch mode powersupply circuit 10 with primary side sensing. The power supply comprisesan AC mains input 12 coupled to a bridge rectifier 14 to provide a DCsupply to the input side of the power supply. This DC supply is switchedacross a primary winding 16 of a transformer 18 by means of a powerswitch 20, in this example an insulated gate bipolar transistor (IGBT).A secondary winding 22 of transformer 18 provides an AC output voltagewhich is rectified to provide a DC output 24, and an auxiliary winding26 provides a feedback signal voltage proportionally to the voltage onsecondary winding 22. This feedback signal provides an input to acontrol system 28, powered by the rectified mains. The control systemprovides a drive output 30 to the power switching device 20, modulatingpulse width and/or pulse frequency to regulate the transfer of powerthrough transformer 18, and hence the voltage of DC output 24. Inembodiments the power switch 20 and controller 28 may be combined on asingle power integrated circuit.

As can be seen, the primary side controlled SMPS of FIG. 1 derivesfeedback information from the primary side of the transformer, using anauxiliary winding to avoid high voltage signals, the voltage beingstepped down by the turns ratio of the transformer. As the skilledperson will appreciate, however, it is not necessary to employ aseparate auxiliary winding although this may be convenient if such awinding is already contemplated to provide a low voltage supply to thecontroller. For example, a voltage of the primary winding may be sensed,preferably capacitor coupled so that it can be referenced to the groundof the controller, and stepped down using a potential divider. Anexample circuit for this is shown inset in FIG. 1, with a dashedconnection to the primary winding 16. The skilled person will furtherappreciate that an auxiliary winding is not necessary to provide a dcsupply for the controller as this may be derived from the high voltagedc supply on the primary side of the SMPS or in a number of other ways,for example using a capacitor charge pump driven via a diode from theswitched voltage on the power switch. In some preferred implementations,therefore, the auxiliary winding is omitted.

We will describe techniques for using the auxiliary voltage waveform togenerate feedback information for regulating an SMPS. In embodimentsthis facilitates operation across a wide range of input and outputconditions.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is therefore provideda switch mode power supply (SMPS) controller for regulating the outputof a discontinuous conduction mode (DCM) SMPS in response to a feedbacksignal from a winding of a magnetic energy storage device forming partof an output circuit of said SMPS, said feedback signal having anoscillatory portion when substantially no energy is being transferred tosaid SMPS output, the SMPS controller comprising: a reference levelinput to receive an output voltage reference level signal; a feedbacksignal input to receive said feedback signal, said feedback signal beingresponsive to a voltage on said winding; a first comparator coupled tosaid reference level input and to said feedback signal input and havingan output responsive to a comparison of said reference level signal andsaid feedback signal; a timing signal generator coupled to said feedbacksignal input to identify a point in said oscillatory portion of saidfeedback signal and to provide a timing signal responsive to saididentification; an integrator coupled to said feedback signal input andto said first comparator output and configured to start integrationresponsive to said comparison of said reference level signal and saidfeedback signal, to provide an integrated feedback signal output; asecond comparator to compare said integrated feedback signal output atsaid time identified by said timing signal generator timing signal witha second reference value, and having an output to provide an errorsignal responsive to said comparison; and a controller output coupled tosaid second comparator output to provide a control signal for regulatingsaid SMPS output responsive to said error signal.

The sensed winding may comprise an auxiliary winding of the magneticenergy storage device or, where the device comprises a transformer, aprimary winding of the transformer.

In embodiments of the invention integration of the auxiliary voltagewaveform is performed from a point determined by comparing the auxiliaryvoltage with a reference voltage. However in broad terms the start pointmay be determined by the actual and/or target operating voltage of theSMPS. Thus the start point can be determined by either comparing thefeedback (auxiliary) voltage with a reference voltage or by finding aknee point in the feedback voltage with a slope detector or transformerflux reset point detector.

The integration may either be stopped at some known, later point orsampled at some known, later point and then, in effect, a comparisonmade between the integrated value and an expected result. Embodiments ofthe invention exploit the observation that in the oscillatory part ofthe auxiliary voltage wave form immediately following the decaying partof the wave form, that is in the post-conduction resonance part of theauxiliary voltage wave form some properties of the areas under the curveof this wave form are known. Thus, for example, over a cycle the totalintegrated area is zero. Similarly the area under, for example, aquarter cycle is known (providing the starting amplitude is known). Thusthe integration may be stopped or sampled at a defined point in thenon-conducting oscillatory phase of the auxiliary voltage wave form andthe effect of having integrated over a part of the oscillatory portionof this wave form may be compensated for or may cancel out, dependingupon the sampling point.

Thus in a first group of embodiments the timing signal generatorincludes a differentiator and the identified point comprises a peak ortrough of the oscillatory portion of the feedback signal, for example,the first trough after the feedback signal has passed through zero. Theintegration may be stopped at this point or, alternatively, the outputof the integrator may be sampled at this point. In this latter case, theoutput of the integrator may be compared with the value it was re-set towhen the integration started, typically zero and a demand signalrepresenting a load on the output side of the power supply generatedaccording to whether the output of the integrator is above or below itsre-set value. This demand signal can be used modulate the pulse widthand/or frequency of a power switching device of the SMPS to regulate theoutput. In a variant, the time at which the integrated output reachesits re-set value, for example zero, the time at which the integratedoutput reaches it re-set value, for example zero, may be employed togenerate the control or demand signal, for example by comparing thiswith a known timing such as the timing signal generated by the timingsignal generator, in described embodiments a trough (or peak) of theoscillatory portion of the feedback signal. In a still further variant,if the integrator is stopped at a peak or trough of the feedback signalthe output of the integrator may be used as an error signal to directlyregulate the SMPS.

In another group of embodiments, rather than the timing signalidentifying a peak or trough of the oscillatory portion of the feedbacksignal, instead a zero crossing of oscillatory portion of the feedbacksignal is identified. This then defines an area under the oscillatoryportion of the curve of one quarter of a cycle or, more generally, of anintegral number of quarter cycles. The area under a quarter cycle varieswith the peak at the start of the quarter cycle, and hence with thedesired output voltage, more particularly with the output voltagereference level signal. Nonetheless this area may be determined andsubstrated from or compared with the total integrated value (to derivean error signal) or, as previously described, the integrator output maybe sampled at this point. In a particularly convenient implementationthe integrator and second comparator are combined to integrate adifference between the feedback signal and the second reference value,and in this case the second reference value may be set equal to ordependent upon the output voltage reference level signal.

In a related aspect the invention also provides an SMPS controllercomprising: a winding signal input to receive a winding voltage waveformfrom a winding of a magnetic energy storage device of the SMPS; anintegrator coupled to said winding signal input to integrate saidwinding voltage waveform; a reference voltage input to receive areference voltage signal; a first comparator coupled to said windingsignal input, to said reference voltage input and to said integrator toreset said integrator responsive to a comparison between said windingvoltage waveform and said reference voltage signal; a timing circuit togenerate a timing signal at a timing point in said winding voltagewaveform; and a control output to provide an SMPS control signalresponsive to a value of said integrated winding voltage waveform atsaid timing point.

The invention further provides a method of regulating the output of aswitch mode power supply (SMPS) operating in a discontinuous conductionmode, the method comprising: monitoring a feedback signal from a primaryor auxiliary winding of a magnetic energy storage device forming part ofan output circuit of said SMPS, said feedback signal including adecaying portion representing a decaying voltage in said output circuitof said SMPS followed by an oscillatory portion when substantially noenergy is being transferred to said SMPS output; integrating saidfeedback signal from a point in said decaying portion of said feedbacksignal defined by a target output voltage for said SMPS; and regulatingsaid SMPS output responsive to a comparison, at a defined point in saidoscillatory portion of said feedback signal, of a result of saidintegrating with a second, reference value.

The invention still further provides a system for regulating the outputof a switch mode power supply (SMPS) operating in a discontinuousconduction mode, the system comprising: means for monitoring a feedbacksignal from a primary or auxiliary winding of a magnetic energy storagedevice forming part of an output circuit of said SMPS, said feedbacksignal including a decaying portion representing a decaying voltage insaid output circuit of said SMPS followed by an oscillatory portion whensubstantially no energy is being transferred to said SMPS output; meansfor integrating said feedback signal from a point in said decayingportion of said feedback signal defined by a target output voltage forsaid SMPS; and means for regulating said SMPS output responsive to acomparison, at a defined point in said oscillatory portion of saidfeedback signal, of a result of said integrating with a second,reference value.

Preferably the second reference value comprises a substantially fixedreference value.

The invention also provides an SMPS including an SMPS controller asdescribed above.

According to a still further aspect the invention provides a powerconverter comprising: a transformer and a switch that electricallycouples and decouples the transformer to and from a power source; and asensing module to indirectly sense an output voltage of said powerconverter and thereby regulate an output of said power converter;wherein said sensing module is configured to integrate part of a primaryor auxiliary winding flyback voltage waveform of the transformer betweentwo points, said two points including a resonant portion of said voltagewaveform, such that when in regulation said part of said waveformbetween said two points has an integrated value of substantially zero.

According to a still further aspect the invention provides a powerconverter comprising: a transformer and a switch that electricallycouples and decouples the transformer to and from a power source; anevent detection module for generating a reference voltage timing signalindicating when a primary or auxiliary winding flyback voltage waveformof the transformer substantially equals a reference voltage; anintegration module for generating a signal indicating when said flybackvoltage waveform has reversed phase starting from a timing pointindicated by said reference voltage timing signal; a critical conductionpoint detector for generating a second signal indicating when a criticalconduction point has been reached; and a phase detection circuit forgenerating an error signal which indicates a relative timing of theoutputs of said integration module and said critical conduction pointdetector.

The skilled person will appreciate that the above-described techniquesmay be employed in a wide variety of SMPS architectures including, butnot limited to, a flyback converter and a direct-coupled boostconverter. In some implementations (as mentioned) the magnetic energystorage device comprises a transformer with primary, secondary, andauxiliary windings but in other implementations an auxiliary winding maybe provided on another inductor of the SMPS. In still otherimplementations an auxiliary winding may be omitted and the sensingsignal derived from a primary winding, for example as described abovewith reference to FIG. 1.

In some embodiments an SMPS controller as described above is implementedmainly using analogue circuitry, in particular for the integration,differentiation, and comparison operations. However, in otherembodiments an SMPS controller as described above may be implementedusing digital circuitry. Thus the invention further provides processorcontrol code, such as RTL or SystemC, in particular on a carrier medium,to define hardware to implement such circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described,by way of example only, with reference to the accompanying figures inwhich:

FIG. 1 shows an example of an SMPS incorporating primary-side sensing;

FIGS. 2 a and 2 b show, respectively, a schematic circuit diagram of anSMPS controller according to a embodiment of the invention, and acircuit for generating a DEMAND signal;

FIG. 3 shows timing waveforms for the controller of FIG. 2, and

FIG. 4 shows an SMPS pulse generator and drive circuitry for use withthe controller of FIG. 2 to implement a flyback SMPS.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

We will describe SMPS control systems which can achieve optimum outputvoltage regulation with primary side feedback while operating across awide range of input and output conditions. Broadly speaking we willdescribe an SMPS controller which integrates a feedback signal from apoint determined by a target operating voltage to a peak or trough of anoscillatory or resonant portion of the feedback signal whensubstantially no energy is being transferred to the SMPS output. Whenregulation is achieved this value should be zero; the difference fromzero can be used to regulate the output voltage of the SMPS.

In more detail, we will describe an apparatus and method for extractingSMPS output voltage information from a primary winding on a transformerof the SMPS. A differentiator differentiates the voltage waveform fromthe primary winding. A first comparator senses the zero crossing of thedifferentiator output, detecting what we refer to as a criticalconduction (CRM) point. A comparator compares the primary voltagewaveform with a reference voltage producing a RESET signal. When theRESET signal goes inactive the integrator integrates the primary voltagewaveform from a pre-defined reset value. A second comparator comparesthe output of the integrator with the reset value, producing a timingsignal. A circuit then compares the phase of the timing signal withrespect to the critical conduction point, producing a logic signalDEMAND, which can be used to control the power supply.

We first describe an operating principle of the SMPS controller.

The controlled SMPS includes a magnetic device and a power switch toswitch power to the magnetic device. The magnetic device has a sensingwinding, which may comprise an auxiliary winding of an inductor ortransformer or a primary winding of a transformer. In order to derivefeedback information from the sensing, say auxiliary, winding waveformthe target operating voltage of the converter and the actual operatingvoltage of the converter are determined. The method indirectlydetermines a mismatch between those two voltages by detecting thecritical conduction (CRM) point in the sensed voltage waveform.

In, for example, a flyback converter, the secondary winding voltage atthe end of the secondary current conduction is equal to the outputvoltage plus the secondary rectifier forward voltage drop. Subsequentlythe residual energy in the transformer will give rise to an oscillatoryvoltage waveform whose resonant frequency is defined by the transformerprimary inductance and associated parasitic capacitance. The area underthe first half cycle of this oscillation, in the auxiliary voltagewaveform, will be zero assuming negligible damping. Therefore if theauxiliary voltage waveform is integrated from the secondary zero current(SCZ) point the integrator will give the first zero crossing at thefirst valley point (i.e. CRM point) of the waveform. That is, referringto FIG. 3, integration from point X to point Y of the auxiliary voltagewaveform (Vaux) will give substantially zero. Moreover a differentiatorwill also produce a zero crossing point at the CRM point as the slope ofthe auxiliary voltage waveform is zero at that point. Using thisprinciple the exact operating point and the target operating point ofthe converter can be found. For example, if the integration begins at apoint before X in FIG. 3, determined by Vref (which sets a desiredoutput voltage), the additional area under the Vaux curve defines theoperating point of the power supply (and the integral will reach zerolater, as shown by Vint).

We now describe an implementation of the above described operatingprinciple in an SMPS controller.

FIG. 2 a shows a schematic circuit diagram of the analogue blocks of thecontroller 200; the timing diagrams are shown in FIG. 3.

Referring to FIG. 2 a, the auxiliary voltage (Vaux) is fed to the RESETcomparator 202, an integrator 204 and a differentiator 206. The RESETcomparator samples the auxiliary voltage waveform at target operatingpoint by comparing it to a reference voltage (V_(ref)) to generate anRESET signal 208. The RESET signal is then used to reset the integratorby means of switch 210 in order to start integrating at the targetoperating point. The integrator output 212 is then fed to a zerocrossing detector 214 (which compares with a zero reference). Thereforethe output of the zero crossing detector will indicate the points whenthe integrator output becomes zero.

The differentiator 206 indicates change in slope along the auxiliaryvoltage waveform. The differentiator output is fed to a second zerocrossing detector 216 that indicates the maximum and minimum pointsalong the auxiliary waveform.

When the error between the actual and target operating points is zeroboth integrator (Vint) and differentiator (Vdiff) outputs will zerocross at the CRM point (see FIG. 3). If the actual operating point isbelow the target operating point the integrator will integrate morepositive area resulting in the first zero crossing point being delayedwith respect to the CRM point (as shown at point t4 in FIG. 3). On theother hand if the actual operating point is above the target operatingpoint the zero crossing point of the integrator will have an early zerocrossing with respect to the CRM point.

The SMPS may be controlled either by the timing of ZCINT, where Vintcrosses zero, or alternatively by the value of Vint at a particulartime, for example t3 in FIG. 3, the CRM point. Both these indicate thedemand made by the load on the output side of the SMPS, and may be usedto control the pulse frequency and/or period of an oscillator driving apower switch of the SMPS.

In one embodiment the ZCINT signal 218 is sampled at the CRM point(given by ZCDIFF 220) and in this way the polarity of the feedback errorcan be identified and, for example, a DEMAND signal generated whichindicates the DEMAND of the converter, as shown in FIG. 3. This singlebit information may, for example, be processed using a single bitoperated digital algorithm to control the power switch of the converter.For further details reference may be made to the applicant's co-pendingapplications PCT/GB2005/050244, PCT/GB2005/050242, GB 0513772.4, and GB0526118.5 (all of which applications are hereby incorporated byreference in their entirety). FIG. 2 b shows an example of a circuitwhich may be employed to sample the ZCINT signal 218 to generate theDEMAND signal (the latch may be reset at any convenient time).

We next describe the timing diagram of FIG. 3 in more detail.

A typical discontinuous mode flyback auxiliary voltage waveform (Vaux)is shown at the top of FIG. 3 followed by the secondary current waveform(Isec). The secondary current becomes zero at t=t₂. The auxiliaryvoltage is sampled by V_(ref) at t=t₀ and t₁. The RESET comparator ishigh during t₀ to t₁ period as V_(aux)>V_(ref). RESET is used to resetthe integrator (V_(int)). Therefore the integrator starts integratingthe V_(aux) only after RESET becomes zero at t=t₁. The area between t₁and t₂ is approximately trapezoidal resulting an approximately linearintegral up to t=t₂. After t₂ V_(int) becomes sinusoidal with a 90°phase lag with respect to V_(aux). This sinusoid is shifted above orbelow zero level depending on when the integration is started. If theintegration is started before t₂ the sinusoid will be shifted above thezero level whereas if the integration is started after t₂ the sinusoidwill be shifted below the zero level. This will determine the positionof the first zero crossing of the V_(int) (at t=t₄) with respect to theCRM point (t=t₃). ZCINT provides zero crossing information of theV_(int) signal to the digital controller.

V_(diff) gives the slope of the V_(aux) at a particular point. Up tot=t₂ V_(diff) is substantially linear. However after t=t₂ V_(diff) issubstantially sinusoidal with a 90° phase lag with respect to theV_(aux). V_(diff) has a zero crossing at t=t₃ irrespective of theoperating point of the converter. ZCDIFF provides zero crossinginformation for the V_(diff) signal to the digital controller.

According to the timing diagram t₃ and t₄ do not coincide. Furthermoreat t=t₃ ZCINT is equal to one. This indicates the actual operating point(at t=t₂) being below the target operating point (at t=t₁) and need foran increase in the output. Therefore (in this example) the DEMAND is setto high at t=t₃.

To now recap the theory of operation, the area under the auxiliaryvoltage waveform starting from the secondary current zero point tocritical conduction point is equal to zero assuming negligible damping.The actual operating point of the power converter (plus a voltage dropdue to the secondary rectifier) can be found at the point when thesecondary current is zero. Therefore if the converter is operating atthe target operating point (voltage) the area integrated starting fromthe actual operating point will be zero at the critical conductionpoint. However if the operating point of the converter is shifted theintegral will not be zero at the critical conduction point.

Moreover the slope of auxiliary voltage waveform will be zero at thecritical conduction point. This leads to the differential of theauxiliary voltage being zero at that point irrespective of the operatingpoint of the converter. Therefore using an integral and differential ofthe auxiliary voltage waveform feedback error of a power converter canbe found accurately.

The controller may implemented at FPGA level for a range of SMPSarchitectures including, but not limited to the flyback converterarchitecture discussed. FIG. 4 illustrates a portion of a flybackconverter architecture 400 which may include an SMPS controller asdescribed above (compare FIG. 1, in which like elements are indicated bylike reference numerals). The flyback converter architecture 400comprises a flyback transformer having a primary winding 16, a secondarywinding 22 and an auxiliary winding 26. The secondary winding isconnected to a rectifier and smoothing capacitor to provide the outputvoltage; the auxiliary winding may similarly be connected to a rectifierand smoothing capacitor to provide power to the control circuitry, aswell as providing an auxiliary winding (“primary-side”) sensing signal.As shown, the secondary winding is usually physically isolated from theprimary and auxiliary windings and their associated components to meetlegislative safety requirements. The DEMAND signal, or another errorsignal derived from Vint as indicated above, provides an input to apulse generator 402 which drives a gate driver 404. The pulse generator402 varies the duty cycle of switching transistor 20 (in this example aMOSFET) by adjusting the timing of the ON and OFF pulses output to thegate driver.

The techniques we have described provide a stable and accurate way ofdetecting the feedback error of a primary side sensing SMPS, with a onlya small number of components in the feedback loop. No doubt many othereffective alternatives will occur to the skilled person. It will beunderstood that the invention is not limited to the describedembodiments and encompasses modifications apparent to those skilled inthe art lying within the spirit and scope of the claims appended hereto.All documents, patents, and other references listed above are herebyincorporated by reference for any purpose.

1. A switch mode power supply (SMPS) controller for regulating theoutput of a discontinuous conduction mode (DCM) SMPS in response to afeedback signal from a winding of a magnetic energy storage deviceforming part of an output circuit of said SMPS, said feedback signalhaving an oscillatory portion when substantially no energy is beingtransferred to said SMPS output, the SMPS controller comprising: areference level input to receive an output voltage reference levelsignal; a feedback signal input to receive said feedback signal, saidfeedback signal being responsive to a voltage on said winding; a firstcomparator coupled to said reference level input and to said feedbacksignal input and having an output responsive to a comparison of saidreference level signal and said feedback signal; a timing signalgenerator coupled to said feedback signal input to identify a point insaid oscillatory portion of said feedback signal and to provide a timingsignal responsive to said identification; an integrator coupled to saidfeedback signal input and to said first comparator output and configuredto start integration responsive to said comparison of said referencelevel signal and said feedback signal, to provide an integrated feedbacksignal output; a second comparator to compare said integrated feedbacksignal output at said time identified by said timing signal generatortiming signal with a second reference value, and having an output toprovide an error signal responsive to said comparison; and a controlleroutput coupled to said second comparator output to provide a controlsignal for regulating said SMPS output responsive to said error signal.2. An SMPS controller as claimed in claim 1 wherein said integrator isresponsive to said timing signal to stop integration.
 3. An SMPScontroller as claimed in claim 1 wherein said error signal has avariable magnitude responsive to a difference between said SMPS outputand said output voltage reference level signal.
 4. An SMPS controller asclaimed in claim 1 further comprising logic responsive to said timingsignal to sample said an output of said second comparator at said timeidentified by said timing signal to provide said error signal.
 5. AnSMPS controller as claimed in claim 1 wherein said timing signalgenerator comprises a differentiator, and wherein said identified pointcomprises a peak or trough of said oscillatory portion of said feedbacksignal.
 6. An SMPS controller as claimed in claim 1 wherein said secondreference value comprises a reset value of said integrator.
 7. An SMPScontroller as claimed in claim 6 wherein said reset value issubstantially zero.
 8. An SMPS controller as claimed in any claim 1wherein said control signal is responsive to a difference between atiming of said error signal and a timing of said timing signal.
 9. AnSMPS controller as claimed in claim 8 wherein said error signalcomprises a logic signal, and further comprising logic coupled to saidsecond comparator output and to said timing signal generator output andhaving an output coupled to said controller output to generate saidcontrol signal.
 10. An SMPS controller as claimed in claim 1 whereinsaid integrator includes said second comparator, and wherein saidintegrated output is responsive to an integration of a differencebetween said feedback signal and said second reference value.
 11. AnSMPS controller as claimed in claim 1 wherein said winding comprises anauxiliary winding of said magnetic energy storage device.
 12. An SMPSincluding the SMPS controller of claim
 1. 13. An SMPS controllercomprising: a winding signal input to receive a winding voltage waveformfrom a winding of a magnetic energy storage device of the SMPS; anintegrator coupled to said winding signal input to integrate saidwinding voltage waveform; a reference voltage input to receive areference voltage signal; a first comparator coupled to said windingsignal input, to said reference voltage input and to said integrator toreset said integrator responsive to a comparison between said windingvoltage waveform and said reference voltage signal; a timing circuit togenerate a timing signal at a timing point in said winding voltagewaveform; and a control output to provide an SMPS control signalresponsive to a value of said integrated winding voltage waveform atsaid timing point.
 14. An SMPS controller as claimed in claim 13 furthercomprising a second comparator to compare said integrated windingvoltage waveform with a second, fixed reference and to generate adigital output representing a result of said comparison, and whereinsaid control signal comprises a sample of said digital output at saidtiming point.
 15. An SMPS controller as claimed in claim 13 wherein saidtiming point comprises a peak or trough of said waveform, and whereinsaid timing circuit comprises a differentiator to differentiate saidwinding voltage waveform and a third comparator to compare an output ofsaid differentiator with zero.
 16. A power converter comprising: atransformer and a switch that electrically couples and decouples thetransformer to and from a power source; an event detection module forgenerating a reference voltage timing signal indicating when a primaryor auxiliary winding flyback voltage waveform of the transformersubstantially equals a reference voltage; an integration module forgenerating a signal indicating when said flyback voltage waveform hasreversed phase starting from a timing point indicated by said referencevoltage timing signal; a critical conduction point detector forgenerating a second signal indicating when a critical conduction pointhas been reached; and a phase detection circuit for generating an errorsignal which indicates a relative timing of an output of saidintegration module and an output of said critical conduction pointdetector.
 17. A power converter as claimed in claim 16 wherein saidcritical conduction point detector comprises: a differentiator forgenerating a signal proportional to the slope of said flyback voltagewaveform; and a comparator for comparing an output of saiddifferentiator output with a reference voltage.
 18. A power converter asclaimed in claim 16 wherein said critical conduction point detectorcomprises: a peak detector for detecting peak or valley points of saidflyback voltage waveform; and a comparator for comparing an output ofsaid peak detector with a reference voltage.